Heavy and large, having a need to exercise (6)
I believe the answer is:
leaden
'heavy' is the definition.
('leaden' can be similar in meaning to 'heavy')
'large having a need to exercise' is the wordplay.
'large' becomes 'l' (**).
'having' says to put letters next to each other.
'to exercise' indicates an anagram (I've seen 'exercised' mean this).
'a'+'need'='aneed'
'aneed' anagrammed gives 'eaden'.
'l'+'eaden'='LEADEN'
'and' is the link.
(Other definitions for leaden that I've seen before include "Leaned over in slow, heavy way" , "Depressing - heavy" , "Dull, heavy, slow" , "Heavy and slow, as if weighted down with heavy metal" , "Slow-moving, being like heavy metal" .)